There are a number of applications in which two or more signals are provided or received by some electronic device, and it becomes necessary to select one of those signals as an output signal. For example, in a typical smart phone, tablet or other device that is capable of functioning as a cellular telephone, an audio and video player, an internet web browser, etc., a user may be listening to music or watching and listening to a video (either stored in the device or streamed from a website, for example) when a telephone call is received.
In such a case it is desirable to pause the music or video or otherwise discontinue listening to and/or watching it, and to instead accept and respond to the telephone call. Thus, the audio of the music or video must be replaced by the audio of the telephone call as the output presented to the user. This is commonly done by means of switches which route the desired signal to the audio output of the smart phone, so that the user may hear the telephone call from the smart phone speaker or headphones connected to the smart phone.
Typically the audio provided by the music player portion of a smart phone will be of a higher sound quality than the audio provided by the cellular telephone portion of the smart phone. Thus, there will typically be a low-quality codec for the telephone audio, as a telephone call is a signal having a low bandwidth and thus resulting in low quality audio, and a higher quality codec for music and for the audio portion associated with at least some video, such as movies.
The low quality codec will also process other low quality signals that may, for example, include only voice audio, such as synthesized voice commands, navigation directions, of the output of “read aloud” applications, and there may similarly be situations where it desirable to switch from the high quality audio to one of these low quality audio signals. The low quality codec typically requires less power than the high quality codec, and thus is used for low quality audio to conserve power and extend battery life of the device. In some instances, the user or the system may choose to have all audio processed by the low quality codec, for example if the device is close to shutting down due to a low power level.
A typical circuit 100 for accomplishing this change of output, such as may be found in a smart phone or tablet, is shown in FIG. 1. In FIG. 1 it is assumed that the low quality telephone audio is received at circuit 100 from the low quality codec (not shown) as Alternate input, and the high quality music or audio portion of a video is received from the high-quality codec (not shown) at the input port having terminals IN− and IN+. The high quality audio is then amplified by an amplifier circuit 102 which, as is known in the art, includes an amplifier 104 and a feedback network consisting of resistors 106. In order to provide a high quality audio output that maintains the audio quality provided by the high quality codec, amplifier 104 is preferably a high performance and low distortion amplifier. (While only one amplifier circuit 102 is shown, there are typically two such circuits for stereo applications, for right and left channels.)
A selection circuit (not shown) generates a signal Select which is used to direct one audio stream or the other to the output port Out, and to switch from one signal to the other at the appropriate time. Which audio stream is selected is typically a function of which application is being run on the device. In circuit 100, signal Select operates switches S1 and S2 to switch the signals provided to the output port Out. Switch S1 operates to feed the high quality music audio from amplifier circuit 102 to Out, while switch S2 operates to feed the low quality telephone call audio to Out. Again, the user may receive the audio output Out either from a speaker in the smart phone or through an attached speaker, such as headphones.
When the user activates the music player or other high quality music source, Select determines the positions of the switches; switch S1 is closed, connecting amplifier circuit 102 to Out, while switch S2 is opened so that no audio from the telephone circuit is provided to Out. When a telephone call is received and the user indicates that the telephone call is to be answered, Select causes switch S1 to be opened, removing the music stream from Out, while switch S2 is closed, providing the telephone call audio to Out.
However, switch S1 is of course an electronic component, and degrades the audio signal that passes through it, and circuit 100 may not perform as well as expected or desired due to the presence of switch S1. It is thus desirable to find a solution for this switching problem which does not require a switch between the amplifier of the high quality audio signal (here amplifier 104) and the output port (here Out).